In recent years, with an increase in capacity, speed or the like of a optical communication network, a substrate for an optical fiber array (hereinafter also referred to as a substrate) is frequently being used as a component constituting an optical splitter which branches one optical fiber into many (e.g., eight) optical fibers, an optical branching filter which branches light in each wavelength region, and even an optical switch which changes light direction, or the like. Such substrate is made of glass, resin or the like, and has an optical-fiber aligning part on which a plurality of grooves (generally V-grooves) retaining and aligning the plurality of optical fibers are formed in parallel.
Conventionally, such a substrate is fabricated by the pressing of glass material (see e.g., Patent Document 1), injection molding of resin material, grinding/polishing of a plate body made of these materials, or anisotropic etching of a plate body made of single-crystal silicon, or the like. However, the above mentioned pressing process requires a precise polish-finishing of the V-grooves so as to obtain a predetermined dimensional tolerance, and thus causes an increase in cost. The grinding/polishing process causes an increase in the number of processes and the complication operation, and hence a decrease in yield. The anisotropic etching is unsuitable for mass production because of high material and equipment cost and a long processing time. Thus, all the methods described above have serious problems and are unsuitable for fabricating the aforementioned substrate.
In order to deal with such problems, Patent Documents 2 and 3 set forth below for example, disclose forming a reduced substrate having a similar shape by heating/softening a preform (glass base material) and then drawing it (also referred to as stretching and forming). Specifically, according to Patent Document 2, the height between the V-grooves at the center portion of the preform and the V-grooves provided at locations other than the center in the widthwise direction differs because of the forming of a recess, at the center portion in the widthwise direction, caused by a slight stretch of the surface of the preform on which the V-grooves are formed, due to heating/softening. In order to avoid such height difference, the preform is formed such that a cross section of the side surfaces (a cross section perpendicular to the longitudinal direction of the V-grooves) presents a projecting R shape, and a through hole is formed within the preform. Patent Document 3 discloses that when heating a preform using a radiation source having at least one infrared radiation apparatus or the like, heat is uniformly applied to the entire preform to be drawn by using a radiation source with a spectral distribution such that at most half of the power is absorbed into the preform while passing through the preform.
[Patent Document 1] JP No. 4-296802A
[Patent Document 2] JP No. 2004-86151A
[Patent Document 3] Japanese Translation No. 2005-505482A of PCT International Application